A liquid crystal light valve projector uses an illumination source to shine light onto a polarizer that reflect S-polarized light to a liquid crystal light valve (LCLV). If the LCLV is fully activated then the LCLV converts the S-polarized light to P-polarized light after it is reflected by a dielectric mirror behind the LCLV. The P-polarized light is reflected back to the polarizer, where it passes through the polarizer to the projection screen. If the LCLV segment is not activated then it does not change the polarization of the light and the light remains S-polarized. The S-polarized light is reflected back to the polarizing surface which in turn reflects the light back towards the illumination source. In a perfect system, 100% of the light reflected from the polarizer to the LCLV is S-polarized light. However, the polarizer does not reflect 100% S-polarized light because of geometrically induced errors in the polarizer. Due to the cone of light that is incident on the LCLV all of the light in the cone is not polarized in the S direction by the polarizer with respect to the LCLV. There are small rotations in polarization direction due to the angle of incidence of the light directed onto the polarizing surface. In this situation a higher degree of twist in the polarization axis occurs for a ray with a higher angle of incidence. The direction of rotation of the polarization away from the ideal is dependent upon the angle of incidence of the ray. For example, a ray incident at 5.degree. might rotate the polarization axis by 2.degree. while a ray incident at -5.degree. would rotate the polarization axis by -2.degree.. This geometrical effect is better described in the Handbook of Optics, Driscoll, Walter G., 1978, McGraw-Hill, p. 1055-1057.
The light rays with polarizations that have been slightly rotated away from the ideal are reflected by the dielectric mirror in the LCLV and return to the polarizer with their polarization axis still slightly rotated. This causes a slight percentage of these rays to plunge into the polarizer instead of being reflected by the polarizer. This small percentage of light is then projected onto the projection screen. Thus, when the LCLV is completely off or inactivated, a small percentage of the light reflected off the LCLV will not be completely polarized in the S direction with respect to the polarizing surface. This causes unwanted light to pass through the polarizer and slightly illuminate the screen causing the "dark" state to be slightly illuminated which reduces the contrast ratio of the projection system.
Another patent mentions placing a retarder in the same position in a projection system. U.S. Pat. No. 4,466,702 by Weiner-Avnear, et al. describes placing a retarder in a similar system to correct for differences in retardation across the color bandwidth of individual channels in a color projector. This patent requires tuning the birefringence plate to each individual spatial light modulator. This is not an effect that the present invention corrects for.
Another problem associated with a dark or off state of the LCLV projector is the problem of birefringence introduced by thermal gradients across the counter electrode substrate. The light valve itself including the substrate is packaged in an anodized aluminum case to reduce reflections that cause unwanted light scattering. Thus, light striking the case holding the LCLV is absorbed creating heat in the LCLV package. This heat is transferred to the counter electrode substrate which causes the temperature to rise in the substrate creating a thermal gradient across the substrate. This thermal gradient creates birefringence in a BK-7 substrate. This birefringence causes the dark state or off state of the liquid crystal light valve to be "lighter" than desired. Therefore, when the light valve is completely turned off, what should be a totally black screen will actually have some light projected on the screen causing the screen to appear gray instead of a deep black. Therefore, it would be desirable to eliminate the birefringence caused by the thermal gradients across the counter electrode substrate and thereby improve the contrast ratio by making the dark or off state of the LCLV appear black instead of gray on the projection screen.
Another aspect of the problem is the type of material used for the counter electrode substrate. The BK-7 material generates mechanical stresses across the counter electrode substrate as the substrate temperature rises. The degree of stress across the electrode is the primary cause of the birefringence in the substrate. It would therefore be desirable to find a material that has a lower coefficient of stress for a given rise in temperature so that there would be a corresponding decrease in birefringence.